The preparation of methylchlorosilanes by direct synthesis (Rochow Synthesis See U.S. Pat. No. 2,380,995), results in the unavoidable formation of a considerable proportion of low-boiling products with a boiling point of less than 40.degree. C. (760 mmHg.). Said low-boiling products include for example, tetramethylsilane, dimethylchlorosilane, methyldichlorosilane, methylchlorosilane, trichlorosilane, methylchloride, hydrocarbons, etc. A method of converting the above-mentioned low-boiling product mixture into compounds (especially trimethylchlorosilane) more useful in the silicones industry is needed.
Many redistribution and alkylation methods are known in the art for the preparation of organohalosilanes from other silanes utilizing aluminum catalysts and reactants, and other materials. Thus, the preparation of organohalosilanes from other silanes has been achieved using a wide variety of catalysts, co-catalysts and reactants such as silica alumina, zeolites, methyl chloride metallic aluminum, hydrogen chloride, aluminum trichloride, methylaluminum halides, etc. Generally, of the Lewis acid activated catalysts, aluminum trichloride was the most widely used for these types of reactions The utilization of aluminum trichloride and organoaluminum compounds as reagents and/or catalysts in these processes however, was fraught with numerous difficulties.
A redistribution reaction is a rearrangement of at least two different substituents which are attached to a silicon atom or atoms Two or more silanes having differing numbers of substituents such as CH.sub.3 or Cl, for example, redistribute when said substituents exchange silicon sites. The resulting product or products still have a total of four substituents or atoms attached to the silicon atom, but in ratios different from that of the starting compounds. A typical redistribution reaction can be illustrated by the following: EQU (CH.sub.3).sub.4 Si+(CH.sub.3).sub.2 SiCl.sub.2 .revreaction.2(CH.sub.3).sub.3 SiCl
A disproportionation reaction is a type of redistribution reaction that occurs when a single silicon compound yields two or more dissimilar silicon compounds having substituents (i.e. CH.sub.3 and Cl) in differing proportions from that of the initial starting compound A disproportionation reaction can be illustrated by the following: EQU 2(CH.sub.3)2SiCl.sub.2 .revreaction.CH.sub.3 SiCl.sub.3 +(CH.sub.3).sub.3 SiCl
The use of organoaluminum compounds in general results in pyrophoric, potentially explosive, and readily hydrolyzable organoaluminum by-products which must be disposed of via lixiviation with water or alkali. This can be extremely hazardous because of the attendant hydrolysis of methylchlorosilanes to yield hydrogen chloride. Aluminum trichloride has appreciable solubility in methylchlorosilane mixtures which can produce separation and purification problems. Plugging problems in transport lines and distillation columns are usually concomitant with its use. Additionally, the use of aluminum trichloride involves long contact times ranging anywhere from two to twenty-four hours.
U.S. Pat. Nos. 2,647,136, 2,786,861 and 3,793,357 all employ the use of an aluminum trichloride catalyst in the preparation of an alkylhalosilane by redistribution. The latter two patents also employ the use of an .tbd.SiH compound as the promoter or co-catalyst thus permitting the use of lower reaction temperatures. All of these patents, however, disclose processes operated under autogenous pressure and at temperatures between 150-400.degree. C. for periods of two to twenty-four hours. Other patents which utilize an aluminum trichloride catalyst with or without a co-catalyst include U.S. Pat. Nos. 2,730,540, 3,655,710 and 3,980,686. It should be noted, however, that not all Lewis-acid catalysts such as "boron trichloride, zinc chloride, iron chloride, copper chloride, etc." exert a "perceptible effect" on the course of these reactions. (U.S. Pat. No. 2,647,136, Column 3, Lines 69-75). Japanese Pat. 3026 (1964) discloses the preparation of phenylmethyldichlorosilane from phenyltrichlorosilane and tetramethylsilane utilizing an activated alumina catalyst at 240.degree. C. for 10 hours in an autoclave. Japanese Pat. 1822 (1957) discloses the preparation of methyltrichlorosilane from trimethylchlorosilane and silicon tetrachloride utilizing an activated alumina catalyst at 280.degree. C. for 4 hours. Contrastingly, Japanese Pat. 23,172 (1961) shows that CH.sub.3 SiHCl.sub.2 is obtained by heating (CH.sub.3).sub.3 SiCl and HSiCl.sub.3 in the presence of alumina at 200.degree.-400.degree. C. under autogenous conditions for up to 10 hours. Though all these patents disclose the use of activated alumina catalysts, the reactions involved require significantly longer reaction times (contact time) typically, of four hours or longer.
The use of methylaluminum sesquichloride to methylate methyl-trichlorosilane is disclosed in U.S. Pat. No. 3,065.253. Here SiH compounds. e.g. methyldichlorosilane, is advantageously used to shorten the reaction time from 20 hours to 2 hours.
U.S. Pat. No. 4,155,927 discloses a process for preparing trimethylchlorosilane by reacting methyldichlorosilane with methyl chloride and metallic aluminum. Methylchloride reacts with aluminum to form methylaluminum sesguichloride. The organoaluminum compound methylaluminum sesguichloride is the methylating agent which reacts with methyldichlorosilane to form aluminum trichloride and trimethylchlorosilane. This reaction is a methylation reaction not a redistribution reaction. This process is not catalytic. Furthermore the production of AlCl.sub.3 results in hazardous waste disposal problems as previously mentioned.
U.S. Pat. No. 4,297,500 discloses a process for synthesizing trimethylchlorosilane from the low-boiling (&lt;40.degree. C.) fraction of the Rochow direct synthesis (U.S. Pat. No. 2,380,995) by hydrochlorination of this fraction in the presence of catalytic amounts of AlCl.sub.3. The amount of HCl employed must be at least equal to the molar amount of tetramethylsilane in the low-boiling fraction. AlCl.sub.3 is disclosed in this patent as a catalyst for the hydrochlorination reaction rather than redistribution.
U.S. Pat. No. 4,158,010 discloses an improved redistribution process for preparing organosilanes by reacting a mixture of alkylhalosilanes with silanes containing an Si-H bond in the presence of organoaluminum compounds and hydrogen halides. The various forms or organoaluminum compounds utilized include: ethylaluminum dichloride, trimethylaluminum, methylaluminum sesquichloride, etc. This patent teaches (see Example 4) the synthesis of trimethylchlorosilane from the low-boiling fraction of the Rochow direct synthesis and added methyltrichlorosilane by heating the reaction mixture at reflux for 6 hours in the presence of methylaluminum sesquichloride and hydrogen chloride. In effect, this method combines hydrochlorination and methylation with redistribution at long contact times.
The process of U.S. Pat. Nos. 3,065,253; 4,155 927; 4,297 500 and 4 158,010 are all hampered by the hazardous handling and disposal problems associated with the use of AlCl.sub.3, organoaluminum halides and gaseous HCl.
U.S. Pat. No. 3,384,652 discloses a method for the production of chlorosilanes and organic substituted chlorosilanes by disporportionation and condensation reactions of mixtures of organochlorosilanes in the presence of crystalline aluminosilicate catalysts (zeolites). These "aluminosilicate materials may also be converted to the H or acid form in which hydrogen ions occupy the cation ion sites" (Column 4, Lines 41-43). In general, "the H form is more stable in materials having SiO.sub.2 /Al.sub.2 O.sub.3 of 3.5 or higher" (Column 4, Lines 45-47). Thus method utilizes zeolites having Bronsted acid sites. Moreover, as in the case of previous processes this reaction requires long contact times.
U.S. Pat. No. 3,207,699 relates primarily to the preparation of catalysts by chemically attaching a restricted quantity of alkylsilyl groups to the internal surface of an acidic refractory oxide of one or more metals (e.g., silica-alumina) at an elevated temperature, cooling the treated acidic refractory oxide in an atmosphere containing no oxygen, whereby the catalytic properties of the acidic cracking catalyst are significantly modified without completely destroying the acidity of the catalyst. Prior to attaching the alkysilyl groups, the refractory oxide is dried at elevated temperatures in order to prevent the reaction of the silane with water because the latter reaction interferes with the desired reaction of the silane with the refractory oxide. This patent discloses that the refractory oxide contains significant cracking activity both before and after treatment with the silane. The treated catalysts so produced are disclosed as being useful as redistribution (specifically disproportionation) catalysts for trimethylsilane. U.S. Pat. No. 3,346,349 relates primarily to the use of treated aluminaceous catalysts, including those of U.S. Pat. No. 3,207,699, as redistribution (specifically disproportionation) catalysts for various silanes. In U.S. Pat. No. 3,346,349 both the class of silanes that can be used to treat the catalysts and the class of silanes that can be redistributed by the treated catalysts are expanded beyond the disclosure of U.S. Pat. No. 3,207,699. U.S. Pat. No. 3,346,349 contains a disclosure similar to the disclosure of U.S. Pat. No. 3,207,699 with respect to the drying of the untreated catalyst and to the method of treatment of the dried catalyst with the silane. U.S. Pat. No. 3,346,349 makes no reference to the cracking activity either of the untreated catalyst or of the treated catalyst. However, in view of the similar treating conditions, it would appear that the treated catalysts of both patents should have similar properties. As is shown in K. Tanabe, Solid Acids and Bases, Academic Press. N.Y. 1970. pp. 123-133, cracking activity in silica-alumina catalysts is associated with the presence of Bronsted acid sites on the catalyst. Hence, accepting the teachings of U.S. Pat. Nos. 3,207,699 and 3 346,349 at face value, it appears they relate to redistribution catalysts with Bronsted acid sites rather than Lewis acid sites. However, it is possible that the silanes reacted with all the Bronsted acid sites on the surface of the silica and alumina in the treatment process of these patents and so the "catalysts" actually become non-acidic during treatment. In view of the high redistribution temperatures actually used in the Examples (e.g. 510.degree. C.), the redistribution reactions reported in these patents may simply have been thermally-induced redistribution reactions as distinguished from catalytically induced redistribution reactions. The possible lack of catalytic activity of the treated oxides of these references (despite the disclosure in U.S. Pat. No. 3 207,699 that the treated catalysts retain significant cracking activity) is also consistent with the fact that the examples of the patents make no reference to any cracking reaction after the initial treatment of the silane. Japanese Pat. 23,172/1961, however, illustrates that methyldiohlorosilane is obtained by heating trimethylchlorosilane and trichlorosilane in the presence of an alumina catalyst at 200-400.degree. C. and under autogenous conditions for up to 10 hours. Longer residence times and/or higher temperatures promote the formation of silane, a known pyrophoric compound.
It is an object of the present invention to provide a method for the redistribution and/or disproportionation of mixtures of organohalosilanes utilizing a heat treated crystalline alumina catalyst.
More specifically, it is an object of the present invention to prepare trimethylchlorosilane from mixtures of other methylchlorosilanes, especially those derived from the lower-boiling fraction of the Rochow synthesis utilizing a heat treated crystalline alumina catalyst
It is an object of the present invention to effect such redistribution and/or disproportionation reaction without the use of aluminum trichloride or other organoaluminum compounds.
It is an object of the present invention to effect such redistribution and/or disporportionation reaction utilizing short contact times.
It is a further objective of the present invention to effect regeneration and reuse of said heat treated crystalline alumina catalyst.